The fiber orientation of fibrous materials is known to often dictate certain properties of the material. The orientation of the cellulose fibers in paper products is an important property to monitor and control during its manufacture.
Fiber orientation is the term used to describe how fibers lay within a sheet of paper. Identifying the direction the majority of fibers are aligned in (Fiber Orientation Angle), and the degree of alignment (Fiber Ratio or Aspect Ratio), characterizes the fiber orientation. The fiber orientation angle is typically a measurement of the angle between the machine direction (MD) motion of the sheet and the principal direction of fiber orientation. The Fiber Ratio is a measurement of the sheet anisotropy (exhibiting properties with different values when measured in different directions) and is the ratio of maximum to minimum fiber distribution 90 degrees apart. An isotropic sheet will have a fiber ratio of one. The Aspect Ratio describes the relative fiber numbers oriented with the fiber orientation angle and perpendicular to the fiber orientation angle.
The fiber orientation of paper is known to dictate important properties such as strength and warp (twist/curl). Strength is particularly important for board (strong boxes) and sack grade (strong bags). Warp is important for box manufacturers where it is desired for the box to be square and for photocopy paper to prevent paper jams. Strength is also important in the printing process since it is generally desired to “pull” the paper through the printing presses as fast as possible. For these reasons paper manufacturers want to optimize fiber orientation in order to maximize strength and minimize warp. Fiber orientation measurement is preferably performed during paper production so that processing conditions can be changed to provide the desired optimum orientation. However, fiber orientation measurements can also be used as a quality control step to determine fiber orientation of the paper obtained as finished product.
Various apparatus for this purpose are known. One method uses a fiber orientation measuring apparatus adapted to detect a differential magnitude of two extreme values of fiber orientation, using a diffusion effect (i.e., light guide effect). This fiber orientation measuring apparatus of well known art detects any extreme fiber orientation appearing in roll paper, particularly running roll paper and/or detects a differential magnitude of two such extreme fiber orientations. In this case, the paper web is irradiated with electromagnetic radiation having a sharp boundary over at least 180 degrees in its cross-section, particularly the laser beam of visible spectrum (visible light). Then, in the vicinity of incident spot on the incident side of the paper web or on the side opposed thereto, the beam transmitted through the paper web is divided into a plurality of predetermined sectors, measured at the position at a predetermined distance from the boundary between the incidence spot and non-incidence region and the measured values in the form of electrical values are compared to each other. To obtain such electrical values, the light beam transmitted through the paper web is optically magnified and guided through an image pick-up lens to obtain an image which is then subjected to photoelectric conversion.
The well known method and apparatus to measure the paper fiber orientation as described above is based on a principle such that a detector is rotated around the optical axis of the incident laser beam to divide this into a plurality of predetermined shapes over the given angle and thereby to obtain a difference or a ratio between the maximum value and the minimum value. However, such method and apparatus have been accompanied with various problems as follow. First, it may be impossible to detect the maximum value and the minimum value at one and same position on the paper web being made since the web is running during rotation of the detector. In order that such apparatus of well known art can achieve the desired measurement, it is essential that the paper fiber orientation to be determined should be constant at any position on the paper web. Regrettably, the fiber orientation more or less depends on the particular position on the paper web being made and therefore it may be impossible for this apparatus of well known art to detect the fiber orientation being variable as the paper web runs. Thus, it may be substantially impossible to, on-line, determine the fiber orientation of the web running through the paper making machine. In other words, it may be impossible to reflect the result of measurement instantaneously in paper being made and thereby to obtain paper having desired fiber orientation.
Additionally, the paper web may be affected by variable running velocity as well as variable diameter of wind-up roll and may shake and/or vibrate in a direction substantially perpendicular to its surface. Consequently, it is difficult to maintain the distance between the paper web and the image pick-up lens constant and thereby to maintain the image in well focused condition. Distinctness as well as shape of the image in the imaging plane may vary due to shaking and/or vibration of the web. Such variation may not be related to an actual variation in the fiber orientation and the intensity of the light beam determined by the detector at a predetermined position for measurement may vary independently of the actual fiber orientation.
In view of the problems as have been described above, it is an object of the present invention to provide method and apparatus to measure fiber orientation of fibrous materials including paper so that the fiber orientation in the fibrous materials such as a paper web being run can be measured on-line reliably. Once provided, the result of measurement can be instantaneously reflected on the paper being made in order to obtain paper of consistently high quality.